1. Field of the Invention
The present invention relates in general to a method and an apparatus for manufacturing a heater roller used for fixing a toner on a recording medium, which heater roller is used on an electrophotographic or electrostatic image forming device. More particularly, this invention is concerned with techniques for controlling the temperature distribution of such a heater roller in the axial direction.
2. Discussion of the Related Art
In one example of such electrostatic image forming device wherein an image consisting of a toner transferred to a recording medium such as a paper sheet or web is fixed on the recording medium under heat and pressure while the medium is passed through a nip between a heater roller and a pressure roller which are provided in the image forming device. In one type of the heater roller, an electrically resistive heat generating layer is formed on the outer circumferential surface of a cylindrical electrically insulating substrate or core. An example of this surface heating type of heater roller is disclosed in JP-A-63-158582. The surface heating type heater roller is adapted such that heat is generated by the electrically resistive heat generating layer which provides the surface that directly contacts the recording medium such as a paper sheet. Therefore, the surface of the heater roller can be heated to a desired operating temperature in a shorter length of time or requires a shorter pre-heating time, than the surface of a conventional internal heating type heater roller in which a heat source such as a halogen lamp is provided in an inner portion of a cylindrical roller body. Accordingly, the surface heating type heater roll has an advantage that the required power consumption is relatively small.
The heater roller of the surface heating type described above is manufactured as disclosed in JP-A-63-158583, for example, by forming an electrically resistive heat generating layer on the outer circumferential surface of a cylindrical substrate or core of an electrically insulating material, by transferring a paste of an electrically resistive material from a suitable transfer sheet to the surface of the cylindrical electrically insulating substrate, or by printing using such an electrically resistive paste. In the former case, the paste may be applied to the transfer sheet by printing in a desired pattern corresponding to the electrically resistive heat generating layer to be formed on the cylindrical substrate. In the latter case, the paste is directly printed on the circumferential surface of the cylindrical substrate by using a suitable printing machine adapted to effect printing on a curved surface. The electrically resistive heat generating layer thus formed on the electrically insulating substrate undesired tends to have an uneven distribution of its electrical resistance (which causes an undesired distribution of its temperature) due to a variation or uneven distribution of the electrical conductivity of the electrically resistive paste and/or the thickness of the electrically resistive heat generating layer formed from the paste by printing. To improve the distribution of the electrical resistance of the electrically resistive heat generating layer once formed, a suitable additional thickness of the electrically resistive paste may be applied to a portion of the initially formed heat generating layer at which the electrical resistance is comparatively high. Further, it is generally known that the temperature of the heater roller is lower at its axially end portions than at its axially intermediate portion, due to an uneven distribution of the heat dissipation in the axial direction. To correct this uneven distribution of the temperature of the heater roller, a suitable additional thickness of the electrically resistive paste may be applied to the axially intermediate portion of the initially formed heat generating layer, so that the axially intermediate portion of the heat generating layer has a comparatively large thickness value.
In the known method of manufacturing the heater roller in which the thickness of the electrically resistive heat generating layer is locally adjusted to assure even axial distribution of the temperature, the circumferential surface of the heater roller has undesirable waviness or stepped sections due to the local addition of the electrically resistive paste. Usually, the electrically resistive heat generating layer formed from the electrically resistive paste by the conventional thick-film forming technique has a comparatively large thickness between 10 .mu.m and 20 .mu.m, for example. Consequently, the force which acts on the recording medium during passage thereof through a nip between the heater roller and the pressure roller tends to be uneven over the surface area of the recording medium, whereby the toner transferred to the medium cannot be uniformly fixed on the medium. This is a problem encountered in the prior art.